The present invention relates to a method of installing a printed circuit board onto an electro-conductive housing, so as to prevent the printed circuit board from emitting electromagnetic wave noise.
FIG. 1(a) and FIG. 1(b) show a conventional method for installing a printed circuit board onto an electro-conductive housing. In order to reduce an electromagnetic wave noise (EMI) emitted from the printed circuit board, the ground pattern formed on a signal layer of the printed circuit board is electrically coupled to the electro-conductive housing of the electronic apparatus.
As shown in the plan view of FIG. 1(a), the conventional printed circuit board is mounted onto the electro-conductive housing by fastening four metal screws through the installation holes located at the four corners of the printed circuit board.
FIG. 1(b) shows a cross sectional view of the installation hole and its peripheral. Since the inner surface of the installation hole is treated with a through-hole metal coating, the ground pattern of the signal layer on the obverse surface and the inner ground layer in the printed circuit board are electrically coupled to the metal post.
FIG. 2 shows a partial plain view of the printed circuit board, in which the inner surface of the installation hole is finished without a through-hole metal coating. In this configuration, the ground pattern of the signal layer on the obverse surface is coupled to the inner ground layer by forming a lot of through holes around the periphery of the installation hole.
As shown in a plain view of the printed circuit board shown in FIG. 3, it has been well known as a conventional technique to increase a number of connecting points between the printed circuit board and the electro-conductive housing, in order to improve an electro-magnetic noise reduction effect.
The electro-magnetic noise reduction effect caused by connecting the ground pattern of the signal layer on the printed circuit board to the electro-conductivity housing lays on the two reasons described as follow:    (1) Since the area of the ground pattern in the signal layer equivalently increases, a capacitance between the ground layer and the earth would increase, resulting in a reduction of voltage fluctuations on the ground layer.    (2) In the printed circuit board having more than four layers, a power source layer, the ground layer and the electro-conductive housing are laminated in an order of the ground layer, the power source layer and the electro-conductive housing. Accordingly, since the power source layer is structurally shielded by the two other layers, the electro-magnetic noises emitted from the gap between the power source layer and the ground layer are reduced. The lager the number of installation points is, the grater the shielding effect is, resulting in a further noise reduction.
By connecting the ground layer of the printed circuit board to the electro-conductive housing, however, sometimes the electromagnetic noises adversely increase. The reasons of this will be detailed in the following.
FIG. 4 is a schematic view, which shows the state that a printed circuit board is installed on the electro-conductive housing.
As shown in FIG. 4, by connecting the ground layer of the printed circuit board to the electro-conductive housing, a resonance would occur in the loop between connecting points. Accordingly, for instance, when the printed circuit board is mounted on the electro-conductive housing by employing the screws arranged in intervals of every 15 cm, the loop distance becomes 30 cm, resulting in the resonance at 1 GHz. Therefore, the noise reduction effect at this frequency cannot be exhibited, but the electro-magnetic noise component residing at this frequency would be adversely emphasized, possibly resulting in an occurrence of great amount of electromagnetic noises.
Generally speaking, since harmonics of clock signals mainly comprise odd-number harmonic components and the energy included in up to ninth harmonics is great, it should be prevented that the resonance occurs at the tenth harmonic frequency component.
In late years, since the clock frequency becomes high and the clock frequency of 100 MHz is generally employed in computer apparatus, it is necessary to set the resonance frequency larger than 1 GHz. To attain this goal, it is necessary to connect the printed circuit board to the electro-conductive housing at an interval smaller than 15 cm. (references: “EMC Design of Print Circuit”, published by Ohmsha)
The technology for solving the abovementioned problem is also set forth in Tokkai 2000-183533.
According to the abovementioned technology, since the ground layers are disposed at upper and lower sides of the power supply layer of the multi-layer printed circuit board, and the upper and lower ground layer are coupled to each other through the through holes established at an outer circumferential area of the power supply layer, the electro-magnetic noises emitted from a gap between the power supply layer and the ground layer can be reduced. However, when this technology is applied to the normal 4-layer printed circuit board, additional layers for the ground layers are necessary and, as a result, a 6-layer printed circuit board would be required. This fact is disadvantageous for its cost reduction aspect.
Further, another technology for improvement is set forth in Tokkai 2001-210922. This technology employs the electro-conductive housing, instead of the second ground layer set forth in Tokkai 2000-210922. According to this technology, in order to connect the ground to the electro-conductive housing, the ground pattern is continuously formed along the outer circumferential area of the outer layer of the printed circuit board, and is coupled to the ground of the inner layer through a lot of through holes, so that the power supply layer is electronically shielded by continuously coupling the ground pattern located at the outer circumferential area to the electro-conductive housing with a metal post or a metal plate. This is a superior technology, which effectively utilizes the electro-conductive housing for the low-cost 4-layer printed circuit board to have the same effect as set forth in Tokkai 2000-210922.
Patent Document 1: Tokkai 2000-183533, Scope of the claims
Patent Document 2: Tokkai 2001-210922, Scope of the claims
Non-Patent Document 1: “EMC Design of Printed Circuit Board” published by ohm publishing Co., LTD.
The prior art of “PRINTED WIRING BOARD AND PRINTED WIRING BOARD IMPLEMENTATION STRUCTURE” set forth in Tokkai 2001-210922 would be an effective technology when the size of the housing is approximately equal to that of the printed wiring board, but is insufficient for the printed circuit board mounted on a big housing.
FIG. 5 shows a figure explaining an outflow of a common mode electric current caused by resonance. FIG. 6 shows a characteristic curve of the electromagnetic noise level versus frequency.
Even if the whole outer circumferential area of printed circuit board is connected to the electro-conductive housing, when the size of the housing is larger than that of printed circuit board as shown in FIG. 5, a resonance phenomenon is generated at a specific frequency f0 by the common mode electric current flowing on the outer surface of the electro-conductive housing as shown in FIG. 6, resulting in a cause of the electro-magnetic noises. In other words, although the electro-magnetic noise emission from the gap between the power supply layer and the ground layer can be eliminated, it is impossible to prevent the electromagnetic noise emission caused by the common mode electric current flowing in the electro-conductive housing itself.